1. Field of the Invention
The present invention relates to a semiconductor manufacturing apparatus and, in particular, to a cold-walled type parallel-flat-electrode plasma CVD (chemical vapor deposition) apparatus.
2. Description of the Related Art
FIGS. 4A and 4B are diagrammatic views showing an arrangement of a conventional cold-walled type plasma CVD apparatus, with FIGS. 4A and 4B showing a plan view and cross-sectional view, respectively, of the apparatus. A plasma SiO film is formed by the apparatus on the surface of a wafer. In the apparatus shown in FIGS. 4A and 4B, reference numerals 11, 12 and 13, 14 represent a reaction chamber made of a metal such as stainless steel, an RF (radio frequency) power source and a pair of electrodes, upper and lower, for plasma generation. The upper electrode contains a heater. The reaction chamber 11 and lower electrode 14 are normally grounded. Reference numerals 15, 16, 17, 18, 19, 20, 21 and 22 show a heater power source, a load-lock chamber (vacuum back-up chamber), a semiconductor wafer, a transfer system, a gas inlet, a reaction chamber exhaust port, an inlet line (N.sub.2 inlet) and a load-lock exhaust port, respectively.
In this apparatus, a predeposition step before loading a wafer into a location of the reaction chamber 11 is carried out to allow a 1.0 .mu.m-thick plasma SiO film to be deposited in the reaction chamber 11. Then a wafer 17 is loaded via the load-lock chamber 16 into the reaction chamber 11 such that it is set on a location, and a plasma SiO film of a predetermined thickness is deposited on the wafer. The deposition of the plasma SiO film is repeated until a total thickness reaches 20 .mu.m (this film thickness is regulated by its uniformity and dust level). At that time, a plasma dry etching is conducted using a CF.sub.4 /O.sub.2 gas and a first cleaning step is performed in the reaction chamber 11. The plasma dry etching steps are repeated six times in terms of the dust level as well as the maintaining of a uniform film thickness, and then a mechanical cleaning--a second cleaning step--is performed whereby the electrodes 13, 14 and inner wall of the reaction chamber 11 are mechanically surface-cleaned.
A cold-walled type plasma CVD apparatus includes a transfer system 18 for loading and unloading a wafer 17 into and out of the reaction chamber 11 and is generally of such a type that, as shown in FIG. 4A, a side wall 11a for allowing access of the wafer 17 to the reaction chamber 11 is provided nearer to the electrodes 13, 14 than the remaining side wall of the chamber, as indicated by a distance l in FIG. 4B.
After the total deposition thickness of the plasma SiO film reaches 20 .mu.m (1.0 .mu.m at the predeposition step +19.0 .mu.m at the batch process), a plasma dry etching is performed, in the CF.sub.4 /O.sub.2 gas stream, at
(1) an amount of flow: 450/50 SCCM; PA1 (2) a pressure: 0.25 Torrs; PA1 (3) a temperature: 300.degree. C.; and PA1 (4) a power output: 2.8 KW
until a 40% overetching is obtained at which time an electric field is concentrated between the electrodes 13 and the side wall 11a at a wafer access opening location as indicated by the distance l in particular in FIGS. 4A and 4B so that an abnormal discharge is produced. In an abnormal discharge zone, a greater amount of polymers including a C--F bond is formed due to an excess presence of F radicals (active radicals) not spent by a plasma SiO etching step, C radicals not consumed by oxygen in the plasma SiO film, and so on, so that the polymers are deposited on electrodes 13, 14 and on the side wall of the reaction chamber. The deposition of the C--F polymers is increased each time a plasma dry etching is effected. As a result, the reaction chamber 11 is placed under an unstable condition and fails to obtain uniform deposition film thickness on the wafer 17.
After the 1.0 .mu.m-thick SiO film was formed at the predeposition step, in order to satisfy the uniformity requirement under the standard (under .+-.5% in the wafer), it is necessary to stabilize the inner conditions of the reaction chamber by performing a given number of plasma dry etchings upon the deposition film thickness of dummy wafer and to perform a mechanical cleaning step at a rate of once per five plasma dry etching steps so that the C--F polymers may be removed from the surface wall of the chamber.
Assuming a throughput of 500 hr/month at an operation rate of 0.8, with 5 plasma dry etchings (etching rate: 800 A/M, 40% overetching rate) and 1 mechanical cleaning step in one process, and a plasma SiO deposition film thickness of 1.0 .mu.m at a predeposition step (deposition rate: 600 A/M) and a plasma SiO deposition film thickness of 19 .mu.m at a batch process (10 wafers can be treated per batch), it is possible to obtain a yield of only about 150 lots per month due to a need to perform a predeposition step.
That is, with the conventional plasma CVD apparatus, an abnormal discharge is generated particularly at a zone between the electrode and the side wall section of the reaction chamber. Under these conditions, the aforementioned C--F polymers are formed on the electrodes and the side wall of the reaction chamber, the chamber is in unstable condition, and the thickness of the depositions is increased. It is necessary to perform deposition steps on a dummy wafer and many mechanical cleaning steps so that the C--F polymers may be removed from the inner wall of the reaction chamber. This causes a marked decline in the manufacturing efficiency.